That doesn’t make sense?
If you make the header position:fixed then there the rest of the page won’t get pushed down and will be hidden behind the fixed element.
You are probably looking at 2 routines. One to push down the page and then a sticky header routine once the page has scrolled. Assuming you are talking about making the revealed element the fixed element?
Of course then you also lose the more/less button as that will slide under the fixed header and not be available until the page is scrolled all the way back to top so are you looking at making that fixed also?
I think you need to be clear about what you are asking for here?